The Er3+ concentration dependencies of upconversion luminescence in oxy-fluoride glass and glass-ceramics containing PbF2 nanocrystals were investigated. Strong red emission from the 4F9/24I15/2 transition was observed with the addition of ~0.8 mol% Er3+ ions, whereas ~10 mol% of Er3+ is required to achieve such emission in several other crystalline hosts. Intensities of red emission further increased with the formation of nanocrystals through heat treatment. The Er3+ ions enriched in glass and segregated preferentially inside the PbF2 nanocrystals that decreased the distance among Er3+ ions and thereby facilitated energy transfer.